Method for evaporating brine

ABSTRACT

A method is provided for evaporating brines, such as sea water, in a system in which the brine is transferred to successive evaporating zones wherein blocking of the transfer lines is avoided. Specifically, the amount of precipitation is limited to avoid slurry viscosities in excess of 150 centipoise and the velocity of the transferring slurry is maintained high enough to keep the slurry particles in suspension. In one specific embodiment, the brine is first treated under atmospheric pressure to evaporate a portion of the liquid therein and to precipitate a substantial amount of solid. The brine is then transferred at its atmospheric boiling point to a reduced pressure flashing zone to evaporate additional liquid while precipitating additional dissolved solid.

United States Patent [72] Inventor George L. Henderson [56] ReferencesCited I g wash- UNITED STATES PATENTS P 2,510,233 6/1950 Kermer 159/3[22] PM 3 219 552 11/1965 S tarm eretal 203/88 x [45] Patented June 22,1971 3248l8l 4 H966 Aklmoto 23/300 [73] Ass1gnees Joseph B. Ward,

Calvin B. Lake, M T Riback George 2,782,097 2/1957 Gostolow 23/273 LX sI 1 Henderson; R v. Lockman' pan interest 3,218,241 11/1965 Checkov1ch..203/11 X mead 3,119,752 1/1964 Checkovich 203/11 Continuation ofapplication Ser. No. FOREIGN PATENTS 532,036,1an. 27, 1966, now PatentNo. 748,572 5/1956 Great Britain 159/47 3,414,483, which is acontinuation-impart of application Ser. No. 233,608, Oct. 29, EEmm".wrNor:mnfYudk0ff 1962 now abandoned sszslant xammer-Jac So erAttorney-Dressler, Goldsmith, Clement & Gordon ABSTRACT: A method isprovided for evaporating brines, such as sea water, in a system in whichthe brine is transferred to successive evaporating zones whereinblocking of the [54] wag fgfi gx BRlNE transfer lines is avoided.Specifically, the amount of precipitamg tion is limited to avoid slurryviscosities in excess of 150 cen- [52] U.S. Cl 159/49, tipoise and thevelocity of the transferring slurry is maintained [59/28, 159/45,159/13, 203/26, 203/48, 23/296 high enough to keep the slurry particlesin suspension. [51] Int. Cl Bold l/22, in one specific embodiment, thebrine is first treated under BOld 1/00 atmospheric pressure to evaporatea portion of the liquid [50] Field of Search 159/13 A, therein and toprecipitate a substantial amount of solid. The 13 B, 2 MS, 28, 48, 45,13, 47, 49, 17, 17 VS; brine is then transferred at its atmosphericboiling point to a 202/173, 182, 236; 203/7, 10, 1 l, 24, 88, 89, 91,reduced pressure flashing zone to evaporate additional liquid 26, 48;23/273, 296 while precipitating additional dissolved solid.

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METHOD FOR EVAPORATING BRINE This is a continuation in part of mycopending U.S. Pat. application entitled, Selected Film VelocityDistillation, Ser. No. 532,036, filed Jan. 27, 1966, now U.S. Pat. No.3,414,483, which in turn, is a continuation in part of copending U.S.application entitled Apparatus and Method for Evaporating Brine," Ser.No. 233,608, filed Oct. 29, I962, now abandoned.

This invention relates to the evaporation of liquids and moreparticularly to a process and an apparatus for the evaporation ofaqueous solutions of solid materials, such as brines.

Common salt, sodium chloride, is a low-cost material in many areas. lnother areas, freight costs add to its price as much as or more than theoriginal cost of the material itself. In such areas, there is sometimesavailable a brine, such as sea water, which might be evaporated, but ingeneral evaporation costs have bee prohibitive.

in some seacoast areas where sunny climates exist it has been possibleto effect solar evaporation economically but solar evaporation requiressubstantial amounts of available land and rising land costs have madesolar evaporation uneconomical in areas which might otherwise be quitesuitable for it. In addition, solar evaporation generally produces nousable condensate and therefore provides no readily available washingmedium and permits only partial purification of the sodium chloride fromother dissolved salts.

Many schemes for the evaporation of sea water have been roposed, bothfor the recovery of its salt content and for the recovery of its water.of these schemes, those involving vapor compression systemsare among themost economical. in a vapor compression system, the vapors evaporatedfrom the brine are heated to a higher temperature by substantiallyadiabatic compression and used at the higher temperature as a source ofheat for the evaporation of additional brine. ln essence, such processesuse compression work rather than direct addition of heat to supply theexternal energy required for evaporation.

While many evaporation systems of this or of a somewhat similar natureare operable, and do serve useful functions in some situations, thereare problems in attempting to employ them more generally in commercialoperations. With a good number of systems, the efficiencies are simplytoo low with the result that unless abundant power can be made availableat a very low cost, the system is simply not economically feasible. Inother cases where a system has been so devised as to obtain betterefficiency, other difficulties have offset that advantage. For example,some of these systems utilize apparatus which requires a capital outlaythat is prohibitive in terms of the expected output of the system. Andsometimes the operation of the system is such that the maintenancebecomes too costly in that it requires frequent replacement of partsand/or servicing of the equipment, possibly along with frequentshutdowns of the system to perform the maintenance thereof.

In evaporation systems wherein the liquid being evaporated is heated byindirect heat exchange with a heating fluid, it is the general practiceto maintain a relatively high temperature differential between the twofluids in order to get maximum utilization of the heating surface of theheat transfer wall. Achievement of this goal, however, is more illusorythan real since high temperature differentials result in uneven heatingand in the development of hot spots on the heat transfer surface. This,in turn, results in the deposition of salts on the heat transfer surfacewith consequent impairment of heat transfer through the wall, thusdefeating the very purpose for which the high temperature differentialwas adopted.

in accordance with one embodiment of this invention, there is provided amethod for evaporating a brine which comprises passing said brinedownwardly as a flowing film over a heat transfer wall, passing a heatedfluid into contact with the opposite side of said heat transfer wall ata temperature no greater than about F. abovethe temperature of saidflowing film, and imparting a downward velocity on said film at theupper heat transfer level of the heat transfer wall at least equivalentto the velocity achieved by a free-falling film of said brine on asmooth vertical wall after a fall of about 3 feet.

It has been found that despite the relatively low-temperaturedifferential maintained in this method, the heat transfer is highbecause of its efficiency. The low-temperature dif ferential preventsthe formation of local hot spots on the heat transfer walls and the highvelocity of the flowing film enhances heat transfer thereto. Heattransfer to a film in turbulent flow is proportional to the velocity tothe 0.08 power.

In addition, the high film velocity prevents the adherence of anyprecipitated salt to the heat transfer surface and instead keeps suchprecipitates suspended in the liquid, thereby avoiding fouling of theheat transfer surface.

In accordance with a second embodiment of this invention, there isprovided an apparatus which has been found to be especially effective inevaporating the brine in the initial stages of the overall evaporativeprocess at which stages the salt content of the brine is not excessivelyhigh. This apparatus is made up of components, most of which can besupplied quite economically and which can be constructed into anoperating system with relative ease. The mode of operation of theapparatus is such that it does not require more sophisticated equipmentseen in many prior art devices (e.g., high-performance equipment whichmay have many moving parts or which must be made to particularly highspecifications since it is to be subjected to high temperatures and/orpressures or other demanding conditions). Concomitant with this is thefact that the problems of maintaining the apparatus in proper workingorder are simplified.

Yet it has been found that this apparatus, while having these variousadvantages, is able to function with high efficiency, not only in termsof relatively high heat transfer coefficients across its heat transfersurfaces, but also in terms of overall efficiency (i.e., power input perunit of fresh water recovered). This is in contrast to many of the priorart devices where to improve the performance of the system it has beennecessary to incorporate more sophisticated devices, which unfortunatelycomplicate the operation of the system and detract from its overalldesirability for a commercial operation.

As a generalization, it may be said that in its preferred embodiment thepresent invention achieves its ends by creating in an especiallyeffective manner a proper balance of the various factors which affectthe efficiency of the system, as well as other practical aspects of thesame. This balance has various facets. For instance, the various factorswhich affect heat transfer across the heat transfer surfaces are broughtinto proper relationship so that the overall heat transfer coefficientis high. Yet this is accomplished in such a manner that there is notimposed on the system burdens disproportionate to the benefits ofachieving these high heat transfer coefficients. (For example, the powerrequirements, the circulating or recycling equipment, the structuralmaterial and other equipment are so arranged and so cooperate with oneanother that they are kept within practical limits.) In the field ofdesalinization especially, where the operability of many differentsystems has long been an accepted fact, but where the economicfeasibility of most all of these systems remains as the paramountconsideration, the importance of this balance throughout the systemcannot be too greatly stressed.

In the present invention, it becomes practical, and in certain aspectsquite desirable, to utilize vertically disposed heat transfer walls of agreater height than was heretofor practical. This, of course, makes itpossible to obtain greater heat transfer surface without using moreground area and without a corresponding increase in circulating and/orrecycling equipment and various structural components. There is thefurther positive benefit that this increased height enhances heattransfer on each wall surface down which the distilland falls by virtueof the fact that the turbulence of the distilland increases as itsvelocity increases. On the opposite side of each heat transfer wall, theheat transfer is maintained at a correspondingly high level by keepingthe thickness of the condensate film within proper limits, this beingvery effectively accomplished by means of traps provided on thecondensate surface at properly spaced vertical intervals.

Another facet of the present invention is that a vapor collectingmanifold is provided at an especially advantageous location at the sideedges of the heat exchange walls. The evaporating process is socontrolled and this manifold is so arranged that it can withdraw thevapor directly from the vaporizing chambers which are defined byadjacent heat transfer walls, without resorting to a separate flashingor settling chamber from which vapor can be collected.

Further, with the evaporating system of the present invention, theconstruction of the evaporating apparatus in a preferred embodimentinvolves essentially the erection of a plurality of vertically disposed,parallel, moderately spaced heat transfer walls, arranged in a manner tocreate alternate evaporating and condensing chambers. Such anevaporating unit can be made essentially in the form of a relativelysimple boxlilte structure, with the component parts thus being able tobe supplied economically and be easily erected.

My invention may be more readily understood by reference to the drawingsof which:

FIG. 1 is a semischematic flow sheet showing the overall process of thisinvention FIG. 2 is a semischematic flow sheet showing the multistageatmospheric evaporation step of the overall process;

FIG. 3 is a semischcmatic cross-sectional view showing the structure ofthe apparatus used in a single stage of the atmospheric evaporation;

FIG. 41 is a cross-sectional view through plane 1-41 of FIG. 3;

FIG. 5 is a fragmentary side elevational view of an evaporating unit ofa second embodiment of the present invention;

FIG. 6 is an isometric view of the apparatus of FIG. 5, with portionsthereof being broken away;

FIG. 7 is a fragmentary sectional view taken on line 7-7 of FIG. 5;

FIG. is a fragmentary horizontal sectional view taken on line 8-8 ofFIG. 7;

FIG. 9 is a fragmentary horizontal sectional view taken on line 9-9 ofFIG. 7;

FIG. 10 is a fragmentary vertical sectional view taken on line 1040 ofFIG. 5;

FIG. 11 is a fragmentary horizontal sectional view taken on line 11-11of FIG. 10;

FIG. 12 is a sectional view, drawn to an enlarged scale and taken online 1242 of FIG. 11;

FIG. 13 is a view drawn to an enlarged scale and detailing generallythat portion of the apparatus circled at 13 in FIG. 8;

FIG. 14 is a sectional view drawn to an enlarged scale and detailingthat portion of the apparatus indicated at 14-14 of FIG. 15, and

FIG. 15 is a fragmentary isometric view illustrating the lower frontportion of the apparatus shown in FIG. 6;

FIG. 16 is a first graph illustrating certain heat transfercharacteristics of the present invention;

FIG. 17 is a second graph illustrating various performancecharacteristics of a specific embodiment of the invention, and

FIG. 18 is a third graph illustrating certain localized heat transfercharacteristics across a heat transfer wall;

FIG. 19 is a graph illustrating the precipitation characteristics of atypical sea water containing 40,000 parts by weight of water, 982 partsof sodium chloride, 44 parts of calcium sulfate and 129 parts ofmagnesium chloride.

Referring specifically to FIGS. 1 and 2, sea water is drawn into thesystem through line 11, passes through pump 12 and into countercurrentheat exchanger 13 where it picks up heat from outgoing condensate and israised in temperature to substantially the boiling point. Generally, theamount of condensate available for heating is somewhat less than theamount of sea water to be heated, and furthermore not all of the excessenthalpy in the condensate can be recaptured. For these reasons anadditional heater 14 may be used if desired to raise the temperature ofthe sea water a few additional degrees in its passage through line 16from the heat exchanger 13 to the atmospheric evaporators 17c-17c. Whenheater 14 is not used, passage of the heated water is through bypassline 15.

In the first of the atmospheric evaporators 17a (FIG. 2) there are aplurality of first stage evaporating chambers 18a into which thepreheated sea water is passed. In evaporation chambers 18a, as will bedescribed more fully hereinbelow, a portion of the water in the brine isevaporated, producing steam which is withdrawn through line 19a andbrine concentrate which is withdrawn through line 21a. The steam iscompressed by compressor 22a to a relatively small pressure incrementabove the atmospheric pressure to which it was generated. Thecompression is substantially adiabatic and the steam is thus raised to ahigher temperature than the temperature at which it was generated, bothbecause the saturation temperature for the steam is higher at the higherpressure and because a small amount of superheat is imparted to thesteam. The compressed steam is passed to desuperheater 23a in which asmall amount of water is injected into the steam to remove its superheatand to bring it to the condensation temperature at its slightly elevatedpressure.

The desuperheated steam is passed into first stage heating chambers241:; which are separated from the evaporating chambers by heat transferwalls 26a. The pressured steam condenses in heating chambers 24a,thereby losing heat content which passes through the heat transfer walls26a and supplies the heat to evaporate the brine in the evaporatorchambers. The condensate produced in the heat exchangers passes out ofthe chambers through line 27a and to return line 28.

The brine concentrate passing out of the evaporating chambers 18a issplit into a recycle portion which passes through line 29a and isblended with the preheated sea water feed and a product portion whichpasses through line 31a to the second stage atmospheric evaporator 17b.The proportion of recycling concentrate to product concentrate may varyconsiderably, as for example, from about zero to about 20 parts ofrecycle concentrate per part of product concentrate and preferably aboutseven to l5 parts of recycle concentrate per part of productconcentrate.

A second stage atmospheric evaporator 17b operates in a manner similarto first stage atmospheric evaporator 17a' with evaporating chambers18b, steam lines 19b, condensate lines 21b, compressor 22b,desuperheater 23b, heating chambers 2%, heat transfer walls 26b,condensate lines 27b, recycle lines 29b, and condensate product lines31b being comparable to the similar elements in the first atmosphericevaporator 17b.

FIG. 2 shows three atmospheric evaporators, the last being 17c (with itscomponent chambers, lines, walls, pump and compressor being designatedon the drawing similarly to the comparable parts of the evaporator 17a,except with the subscript 0). In an actual embodiment it is preferred touse at least four stages of atmospheric evaporation and up to about 20to 30 stages. In the last stage a sodium chloride recycle stream, from asource hereinafter specified, is blended with the brine concentratefeed. The condensate from atmospheric evaporator 17c passes through line27c to condensate manifold 28 and thence through heat exchange 13 ashereinbefore stated. After losing its heat in the heat exchanger thecondensate is passed through line 32 to condensate storage.

The product concentrate which is passed out of the last atmosphericevaporator through line 310 preferably has had at least 70 percent ofits water removed and most preferably from about 72 to about 76 percentremoved. At this concentration, under the conditions of the invention,there is a considerable amount of precipitated salt in suspension in thebrine, almost enough to thicken the brine to the point where wall filmflow thereof cannot be maintained.

FIGS. 3 and 4 show a single atmospheric evaporator, such as the firststage evaporator 17a and shows its general structure. FIG. 3 shows a setof four evaporating chambers 18a and a set of three heating chambers240. It is preferred that at least .six each of heating and evaporatingchambers be supplied for the first atmospheric evaporator 170 with thenumber decreasing for subsequent evaporators as the volume of liquiddecreases.

in the atmospheric evaporator the heated sea water in line 16 isdistributed to the several feeder lines 33a to distributors 34a situatedabove heating chambers 24a and separated therefrom by barriers 36a. Thesea water passing into the distributors 34a spills over the serratededges 370 at the top of walls 380 which are upward extensions of heattransfer walls 26a. As the sea water spills over the serrated edge, itforms a film 35a on the wall extension 384 which flows downwardly overthe wall extension and over the heat transfer wall therebelow. Theupward extensions 380 are of sufficient height to assure the desireddownward velocity of the flowing film by the time it passes the level ofbarriers 36a and flows onto the heat transfer wall 260. The heightnecessary to achieve the desired theoretical velocity may be calculatedfrom the well-known formula wherein v is the desired velocity, v, is theinitial velocity, a is the acceleration due to gravity (32 feet persecond) and s is the vertical height. Since v, is usually zero, it maybe seen that the minimum desired theoretical velocity of 14 feet persecond requires that the height of the extension be at least about 3feet. Preferably, the extension is about 5 feet high, producing atheoretical velocity of about 18 feet per second. Of course, the actualvelocities achieved are short of these theoretical velocities; but theactual velocities are very difficult to ascertain with any great degreeof accuracy and can vary, depending upon the viscosity of thedistilland, etc. Hence, these theoretical velocities are presented heremerely as an approximation of comparison of what the actual velocitiesmay be for different height wall extensions. However, it is to beunderstood that regardless of the precise velocities actually achieved,such 3 and 5 foot wall extensions have proven to be generallysatisfactory in the present invention.

In FIG. 3 the total height is designated as the dimension L and theheight of the upward extension is designated as the dimension L.

As the film of sea water progresses down the heat transfer wall 26a, itincreases its downward velocity. It is preferred that the total heightof heat transfer wall 26a-and its upward extension 3811 be at least 16feet so that the theoretical velocity at the bottom of the heat transferwall shall be a least 32 feet per second. Preferably, the total heightis between about 20 and about 24 feet. The flowing film becomes thinneras it flows down the heat transfer wall both because of its increasedvelocity and because of the evaporation of a portion of the watertherein. The film thickness at the top of the heat transfer wall isgenerally in the range of about 0.01 to about 0.1 inches in thickness.

The flowing film picks up heat from the heat transfer wall' and aportion of the water therein is evaporated. During a single pass overthe heat transfer wall a relatively minor proportion of the watercontent (of the order of about 5 to about 20 percent, preferably aboutpercent) is evaporated. The amount of evaporation in a single pass islimited in order to avoid a substantial change in the boiling point ofthe brine in any one stage. The brine, being more concentrated at thebottom of the heat transfer wall, has a somewhat higher boiling pointthan at the top. For this reason, it is preferred to concentrate thebrine in at least four stages ofatmospheric evaporation, and possibly asmany as 20 to 30 stages.

The heat transfer walls are generally flat plates for simplicity ofconstruction. l6-gage aluminum sheet may be used.

The distance between the heat transfer walls is dependent upon the rateof evaporation of the liquid. From sea water about 3 to about 10 poundsof water per hour are evaporated per square foot of heat transferwall.At such rate of evaporation, the heat transfer walls are generally about2 to 3 inches apart.

The vapors produced by evaporation of water from the brine flow arepassed out of the evaporation chambers through lines 390 which lead toline 19a. The stream is passed to compressor 22a which adiabaticallycompresses it to a pressure increment of about 1 to 5 p.s.i., therebyraising its temperature by about 10 F. to about 50 F. The temperatureincrement includes a superheat increment of about 7 F. to about 35 F.which is removed in desuperheater 23a by the injection of a small amountof water, usually less than about 1 percent of the weight of thesuperheated steam. The desuperheated steam at a temperature betweenabout 3 F. and 15 F. higher than its highest temperature of generationat the bottom of the evaporating chamber is then passed into the heatingchambers 240 through headers 412: containing openings 400. As thesaturated steam loses heat to the heat transfer walls 26a, it condenseson the walls and flows downwardly as a film on each wall. In order toavoid buildup of the condensate films to a thickness which would impedeheat transfer therethrough, collectors 42a are provided at spacedvertical locations on the walls. It is usually desired that thethickness of the condensate film be limited to not more than about 0.005inches. Spacing'of the condensate collectors at a vertical displacementfrom each other of no more than about 4 feet, and preferably no morethan about-2 feet, achieves this result.

The condensate collectors 42a slope downwardly, as shown in FIG. 4toward the downcomer lines 43a which connect with line 27a and thence tocondensate line 28.

Although the temperature differential between heating chamber and theevaporating chamber is quite low, excellent heat transfer is achieved bya combination of several factors, including the limitation of thethickness of the condensate film, the high velocity of the brine filmand the avoidance of fouling on the evaporating side of the heattransfer wall by the low-temperature differential and by the fast movingbrine film.

The concentrated liquor is removed from the evaporating chambers throughlines 44a which lead to line 210 which in turn leads to recycle line 29aand to line 31a which feeds the concentrate to the next set ofatmospheric evaporators. The brine is passed through the desired numberof stages with the precipitated salts carried in suspension by the highliquid velocity. The concentration of precipitated salts rises with theevaporation of water from the brine until the brine concentrateapproaches a viscosity beyond which it cannot be maintained as a flowingfilm. For sea water containing about 2.38 percent of sodium chloride,0.31 percent of magnesium chloride, and 0.1 1 percent of calciumsulfate, the brine is concentrated until about 70 to 76 wt. percent ofthe water has been removed.

At this concentration most of the sodium chloride is still in solutionbut a substantial amount of calcium sulfate is precipitated.

The brine concentrate leaving the last of the atmospheric evaporatorsthrough line 31c is preferably passed through filter 46 prior to beingintroduced into the first flash chamber 470. Alternatively, the filermay be bypassed by line 48. Calcium sulfate is removed from filter 46and is passed to slurry mix tank 51, as shown schematically by lie 49.

In flash tank 47a a moderate vacuum is applied to the brine concentrateto cool it by lowering its boiling point. The cooling in flash chamber470 is substantially adiabatic. Constant stirring is maintained in flashchamber 470 by means of stirrer 49a. The brine concentrate from thefirst flash chamber is removed through line 52a and passed to the secondflash chamber 47b. Generally, a vacuum of the order of about 15 inchesof mercury is applied in the first vacuum flash chamber. The number ofvacuum flash chambers may vary suitably from about two to about fourwith three being showtiin FIG. 1. 1n the last vacuum flash chamber 470 areduced pressure of about 20 to 24 inches of mercury is maintained sothat the temperature therein is of the order of about to about F. Theadditional water evaporated from the flash chambers is passed outthrough lines 53a, 53b, and 53c to header 54 from which it may becombined with other steam generated in the process or may be utilizedfor heating purposes in the process.

In the flashing chambers the total temperature drop is about 100 F.(from about 250 F. in the last atmospheric evaporating chamber to about150 F. in the last flash chamber) making about 100 B.t.u. per pound ofconcentrate available for additional evaporation. This amount of heatwill evaporate about 10 percent of the remaining water in theconcentrate passing to the flash chambers.

The brine slurry from the last flash chamber 47c is passed through line52 c to filter 56 from which the calcium sulfate cake is withdrawn toslurry mix tank 51 as shown schematically by line 57 while the filteredconcentrate passes through line 58 to the evaporative crystallizer 59.At this point, about 80 to 85 percent of the weight of the original seawater has been removed so that the filtered concentrate is about topercent thereof.

This first cut of separating the suspended crystallized salts from thesaturated solution removes the major quantity of the calcium sulfatewith an attendant very small proportion of the total sodium chloride.This relationship of solubility and resultant precipitation as water orsolvent is removed is shown on FIG. 19.

Washing the precipitated slats on the filer and recycling the wash waterreclaims a substantial portion of the amount of sodium chloride that isprecipitated with the calcium sulfate. This method produces asubstantially pure calcium sulfate and returns most of the sodiumchloride to be recycled finally emerging as product.

Continuous high-velocity motion of the slurry is used all during theprocess from the last atmospheric evaporator through the flash tanks andcomplete filtration to eliminate the possibility of stoppage of pipelines or buildup of salt crystals in tank bottoms.

Throughout the process, the amount of precipitation is controlled sothat the viscosity of the slurry carrying the precipitate is not higherthan about 150 centipoisc under operating conditions. At the same time,the velocity of the slurry is maintained at a high level, preferably inexcess of about 8 feet per second, by the use of relatively high pumpingpressure and the use of small transfer pipes.

It has been found that the combination of high velocity and limitedviscosity at these levels avoids stoppage of the pipe lines.

The precipitated salt in the slurry mix tank from filters 46 and 56 isstirred with condensate water introduced through line 61. The salt isreslurried and passed through line 62 to filter 63 from which calciumsulfate is taken off as indicated by line 64. Filter 63, as well asfilters 416 and 56 described above, are preferably continuous rotaryfilters, such as Oliver filters, with a doctor blade continuous removingthe filter cake.

l-Iot wash water introduced through line 66 selectively redissolves thesodium chloride from the filter cake while precipitating calcium sulfatefrom the liquor on the cake. The filtrate from filter 63 is recycled tothe atmospheric evaporation step, andparticularly to the lastevaporating chamber 18c through line 67, as shown in FIGS. l and 2. Theamount of sodium chloride in the recycle stream recovered from filter 63is generally between about 10 and about 30 percent of the originalsodium chloride content of the sea water.

In the first evaporative crystallizer 59 the pressure is reduced toabout 2.6 inches of mercury absolute and a temperature of about 130 F.is maintained. Additional evaporation takes place and small crystals ofsodium chloride. called seed crystals" are formed in the filtrate. Theslurry of seed crystals is passed through line 63 to a secondevaporative crystallizer 69 in which a pressure of about 3 inches ofmercury absolute and a temperature of about 145 F. are maintained and inwhich the seed crystals grow while additional evaporation takes place.The slurry thenpasses through line 71 to the third evaporativecrystallizer 72 which serves as a cooling crystallizer in which crystalsize is built up to about 24 mesh.

In the cooling crystallizer a pressure of about 2 inches of mercuryabsolute and a temperature of about F. are maintained. The first andsecond evaporative crystallizers are heated, as is well known in theart, by continuous withdrawal of a portion of the contents, passagethrough heat exchangers (not shown) and recycle.

At this point, as shown by the solubility curve of FIG. 19, the majorportion of the sodium chloride has been crystallized and precipitatedwith a very small amount of calcium sulfate and essentially zero percentof magnesium chloride. The separation is purposely made at aconcentration which leaves a mother liquor or bitterns containing about10 percent of the total sodium chloride still in solution together with100 percent of the magnesium chloride. Control of the amount of waterevaporated up to this cut permits control of the purity of the productsodium chloride. A partial recycle of the bitterns can be used toreclaim some of the sodium chloride if economically warranted.

To summarize, control of the ratio of salts precipitated and removed ateach step combined with (l) washing the filtrate to increase purity ofthe product and (lb) recycling the wash liquors and mother liquors willproduce products of the purity required.

The concentrated slurry from the cooling crystallizer is passed throughline 73 to centrifugal separator 74, in which the precipitated sodiumchloride is separated from the concentrated liquor which contains sodiumchloride and magnesium chloride. The solids recovered on the centrifugalseparator are washed with hot water (about F.) introduced through line75, and the crystals are reduced in size from about 24 mesh to about 30mesh. The washing removes substantially all of the calcium sulfate fromthe sodium chloride product. Removal is not effected by dissolution ofthe calcium sulfate, but rather by physical transport in the wash watersince the calcium sulfate is flocculent while the sodium chloride isrelatively dense.

The washed crystals comprising substantially pure sodium chloride arepassed to sodium chloride storage as shown schematically by line 76. Theconcentrated liquid, called bitterns" is passed through line 77 with aportion being recycled through line 78 to the first evaporativecrystallizer. Generally, the proportion of bitterns recycled is betweenabout 25 percent and about 75 percent of the total bitterns removed fromthe centrifugal separator. The portion of bitterns which is not recycled(about 5 percent by weight of original sea water) may be subjected totreatment to recover magnesium salts and other materials therefrom, asis well known in the art, or may be discarded.

The overall process is quite economical with respect to heat inputrequirements. While the boiling of 100 pounds of water by direct heatinput requires about 100,000 B.t.u. the method of this inventionrequires compression power equivalent to only about 5,000 Btu. for thesame amount of evaporation. The power requirements of the pumpingoperations are relatively low, of the order of about 10 B.t.u. per 100pounds of water evaporated.

While the invention has been described above with particular referenceto the recovery of sodium chloride from sea water, it is to beunderstood that it is generally applicable to the recovery of eithersolvent or solute in any solution of a solid in a liquid. The inventionis useful in the evaporation of brines other than sea water for recoveryof either the salt or the water therein, or both. The invention isuseful in the evaporation of saline solutions of low sodium chloridecontent, such s brackish waters, or in the evaporation of salinesolutions of high sodium chloride content, such as the waters of saltlakes or inland seas.

FIGS. 5 through 15 illustrate an evaporating unit 1100 of a secondembodiment of the present invention, especially adapted for the recoveryof solvent. As indicated previously, the embodiment illustrated by thisunit 100 has been found to be especially effective in evaporating seawater in the initial stages of the overall evaporating processillustrated in FIGS. 1

and 2, during which stages roughly about half of the water isevaporated. Thus, for example, this unit 100 can well be utilized as theevaporator indicated at 170 in FIG. 2. Since the operation of the units170, b and c have been described previously herein, it is believed thatthe operation of the apparatus shown in FIGS. through 15 will be readilyunderstood by now describing the construction of the same and indicatingonly generally the function of its component parts.

This evaporating unit 100 can be seen to have a general boxlikeconfiguration and thus comprises front and rear vertical walls 102 and104, respectively, and two verticalsidewalls 106. These walls 102, 104and 106 are closed by top and bottom covers I08 and 110, respectively,to form a substantially closed, boxlike structure. Extendinglongitudinally between the front and rear walls 102 and 104 are fixedlysecured thereto, are a plurality (six in the specific apparatus shownherein) of vertical heat exchange walls 112, these walls being disposedin general parallel relationship with one another and with the sidewalls106. The top and bottom edges 114 and 116, respectively, of each of thewalls 112 terminate a moderate distance (e.g., about a half foot) shortof, respectively, the top and bottom cover plates 108 and 110, so thatthere is provided in the unit 100 an upper chamber 118- for the infeedapparatus for the distilland which is to be passed down the heatexchange walls 112, and a lower collecting chamber 120 to collect thedistilland as it drops from the heat exchange walls 112.

Alternate adjacent pairs of the heat exchange'walls 112 are providedwith top and bottom horizontal closure plates 122 and 124, respectively,so that these altemate pairs of heat exchange walls 1 12, with theirrelated plates 122 and 124 and with the portions of the front and rearwalls 102 and 104 located therebetween, define closed chambers 126 whichfunction as vapor condensing chambers. The spaces between the remainingpairs of heat exchange walls 112 are open both top and bottom, and thusdefine chambers 128 which are in free communication with theaforementioned upper distilland intake chamber 118 and the lowerdistilland collecting chamber 120. These chambers 128- function asevaporating chambers and are located alternately between the condensingchambers 126. Also, each of the two side walls 106 defines, with itsadjacent heat exchange wall 112, a related one of the evaporatingchambers 128.

Extending a short distance upwardly (e.g., about two inches) from thetop edge 114 of each heat exchange wall 112 is a related one of severalflange members 130. Alternate pairs of these flange members 130, withthe related plate 122 extending therebetween, define a related one ofseveral distilland distributing troughs 132 (see FIGS. and 11), each ofwhich is located in the upper distilland intake chamber 118 immediatelyabove a related condensingchamber 126.. To distribute distilland to eachof these troughs 132, there is provided at the upper rear end of theunit 100 a distilland intake manifold 134 which receives the distillandthrough a single intake pipe 136 and feeds the distilland into severalintake openings 138, each of which is provided at the rear end of arespective one of each of the troughs 132. Each trough 132 has a sectionof screen 140 located therein to dissipate any turbulence in thedistilland flowing from the opening 138 into the troughs 132. This isconveniently accomplished by arranging each section of screen 140 in aroll of one or more layers, the diameter of the roll of screen beingapproximately the same as the width of its trough 140, and with the rollof screen 140 lying lengthwise in its related trough 132. The distillandflows in through the intake manifold andthrough the openings 138 intothe several troughs 132, from which the distilland falls in a film downthe evaporating surface 142 of each of the heat exchange walls 112. (Theevaporating surface142 is, of course, that wall surface facing itsrelated evaporating chamber 128.) The distilland which falls from thebottom edge 116 of each wall 112 is collected in the lower chamber 120and passes out a pipe 144 to be further processed in the mannerindicated previously herein.

The vapor which evaporates from the falling films of distilland in theevaporating chambers 128, is withdrawn from the chambers 128 by means ofa vapor collecting manifold 146 (see FIGS. 6, 9 and 15) located at thelower front portion of the evaporating unit 100. This manifold 146 has ageneral boxlike structure; its width is the same as that of theevaporating unit 100; and its height in the particular apparatus shownherein is about one-third to one-quarter that of the entire evaporatingunit 100. So that the manifold 146 can communicate properly with theevaporating chambers 128, those portions of the front wall 102 which liewithin the manifold 146 and are adjacent the evaporating chambers 128are entirely cut away to form vapor outlet openings 148 of a relativelylarge cross-sectional area. Thus, vapor passes from the chambers 128through these openings 148 into the manifold 146; and the vapors passfrom the manifold 146 through an outlet conduit 150 to a compressor (notshown in FIGS. 5 through 15).

As indicated previously in the general description of the process of thepresent invention, these vapors are compressed substantiallyadiabatically, then desuperheated by the addition of water, and thenpassed into the condensing chambers,

which in the present embodiment are designated 126. To feed thecompressed vapors into the condensing chambers 126, there is provided atthe upper front portionof the unit an infeed manifold 152 (see FIGS. 5,6 and 8) having a general boxlike configuration. The compressed vaportravels into the manifold 152 through an intake conduit 154, and passesfrom the manifold 152 into the uppermost portion of the condensingchambers 126 through openings 156 formed in those portions of the frontwall 102 which are within the manifold 152 and adjacent the condensingchambers 126.

As previously described herein, the compressed vapors condense in thechambers 126 on the heat exchange wall surfaces 158 (i.e., condensingsurfaces) which face the condensing chambers 126.

As previously described herein, the compressed vapors condense in thechambers 126-on the heat exchange wall surfaces 158 (i.e., condensingsurfaces) which face the condensing chambers 126.

Condensate traps 160 (see'Figures 5, 6, 12 and 14) are provided on eachof the condensing surfaces 158 at vertically spaced intervals of onefoot. As illustrated in H0. 12, each condensate trap.160 comprises anupper flange portion 162 by which it is joinedto its related heatexchange wall 112, and a lower trough portion 164 to collect thecondensate formed on the heat exchange surface portion 158 thereabove.Each such trap 160 has a moderate rearward slope so that the condensatecollected therein flows to the rear of each condensing chamber 126 andfalls to the bottom of the condensate chambers 126. A manifold 166 isprovided at the lower most end of the condensing chambers 126 to collectthe condensate from the lower ends of the changers 126. This manifold166 has an outlet pipe 166a and a pressure relief valve, indicatedat-166b.

To provide lateral support for the several heat exchange walls 112,there are located in each condensing chamber 126 a plurality of bracingplates 168. Each of these bracing plates 168 is conveniently attached toits two adjacent heat exchange walls 112 by formingthe two edge portionsof each plate 168 as downturned lips 170 which engage the upturned edgeof the related condensate trough 164 (see FIG. 12). Most of the area ofeach plate 168 is formed as through openings 172 so that the steam canpass freely down through the condensing chambers 126. Brackets 174 aresecured to the top of the unit 100 so that support from an overheadlocation can be provided for the unit 100.

While the mode of operation of this unit 100 should be readily apparentfrom this detailed description of the same, when read in light of theprevious description of the overall process of the present invention, itis believed the following comments will be of further aid inunderstanding the merits of the present invention.

llll

With regard to the vapor-collecting manifold M6, the crosssectional areaof the openings Mb should be sufficiently large so that the vapor doesnot have a very high velocity as-it leaves the vaporizing chambers 12%.Otherwise the vapor will become contaminated with droplets of the brine.Experimental results indicate that if the velocity of the vapor throughthe openings M8 does not exceed roughly about two feet per second,relatively pure vapor can be collected. If it is desired to so build theapparatus to extend the heat transfer walls laterally, in order to keepthis vapor velocity within the desired limits, the manifold M6 can beextended upwardly; and a second vapor-collecting manifold can beprovided at the rear of the unit MM).

In the apparatus disclosed in the first embodiment of the presentinvention, provision was made for imparting an initial downward velocityof the falling film of brine as it comes into contact with the heattransfer surface. As indicated previously, this becomes especiallycritical when handling brine of higher salt concentrations. However,with lower salt concentrations, it is possible to pass the falling filmonto the heat transfer walls with a very low initial velocity and thusutilize substantially all of the wall area for heat transfer.

To examine experimentally the heat transfer characteristics of the heattransfer walls as used in the present invention, a single heat transferwall was erected. It was 15 feet in height and was made of l8-gagealuminum sheeting. On the condensing side of the sheet, seven collectinggutters were provided at 2-foot intervals. Each side of the wall wasenclosed with a respective boxlike structure so as to form anevaporating chamber and a condensing chamber, generally similar to thoseshown previously herein. Relatively pure water (i.e., tap water, whichhas essentially the same heat transfer characteristics as sea waterwhich has not been concentrated to an appreciable extent) was raised to212 F. and passed down the evaporating side of the wall with thepressure in the'evaporating chamber being atmospheric. Saturated steamhaving a pressure of about 1 pound per square inch gage was fed into thecondensing chamber. Vapor was removed from the evaporating chamber, andcondensate was collected from each gutter separately by means of aplurality of hoses (each leading from a related gutter) so that the heattransfer characteristics could be determined for each wall portionbetween a related pair of proximate gutters. A number of runs were madein which the flow rate of water and the temperature differential betweenthe condensing chamber and the evaporating chamber were each variedindependently. The results of these runs were tabulated and aresummarized in the accompanying graph of FIG. 16.

In the graph of FIG. 16, curve A is a plot of the overall heat transfercoefficient for the entire heat transfer wall (measured in terms ofB.t.u.s per square foot per hour per degree Fahrenheit temperaturedifferential) when the temperature differential is varied and the flowof water is maintained at a substantially constant flow of 1 1.2 gallonsper minute for each lateral foot of the heat transfer wall. Curve B is aplot of the overall heat transfer coefficient for the entire heattransfer wall when the temperature differential was held substantiallyconstant at 3 F. and the flow of water down the heat transfer wall wasvaried. In general, it can be seen that the overall heat transfercoefficient was relatively high for temperature differentials between 2and 4 F., and also relatively high for flows which varied between 7.5and ll.5 gallons per minute per lateral foot of wall. Further, it wasfound that the heat transfer coefiicient for each wall section betweeneach adjacent pair of gutters was high. Experimental results other thanthose used to draw the graph of FIG. 16 indicate that heat transfercoefficients yet higher than those indicated in the graph of FIG. 36 canbe attained in practicing the present invention.

To examine further the operating characteristics of the presentinvention, an evaporating unit was constructed substantially identicalto that shown in FIGS. 5 through 15. The heat transfer walls 112 werefabricated of l8-gage aluminum sheeting; these were made 15 feet longand 2% feet wide, and were spaced 3 inches from one another. The unitwas operated in the manner described previously herein, using freshwater as the distilland. Based on data taken during the operation ofthis unit, the graph of FIG. 17 was prepared to show the operatingcharacteristics of the unit. In the graph, the pounds per hour indicatedis the amount of water condensed from the unit; the C.F.M. indicates thecubic feet of vapor per minute evaporated from the unit; AT F."indicates the temperature differential in degrees Fahrenheit, and APp.s.i. indicates the pressure differential in pounds per square inchbetween the condensing and evaporating chambers. From this data, it isbelieved that those skilled in the art will be able to select theoperating conditions best suited for a particular operation (e.g., interms of desired output, overall economy in view of power costs, etc.)For example, in an area where power is inexpensive relative to the costsof installing the apparatus, it would be better to operate at the upperend of the curves. On the other hand, if opposite conditions prevail, itwould be better to operate at the lower end of the curves.

It is to be understood, of course, that if a unit such as this were tobe made for a large commercial operation, the number of heat transferwalls would be increased; and quite possibly the lateral dimension ofthe heat transfer walls would be increased. The number of stages usedwould depend on various things, such as whether or not it is desired torecover salt from the brine, etc.

In the unit shown in FIG. 5 through 15, the condensate collectinggutters were spaced vertically 1 foot apart. As indicated previously,this arrangement of vertically spaced gutters is considered to beespecially significant in the present invention in obtaining a properheat transfer balance. Perhaps this can be best understood withreference to FIG. 18, wherein is shown the heat transfer characteristicsat specific locations over a 16-foot vertical wall where there is a freefalling film on one side of the heat transfer wall, and steam condenseson the opposite side of the wall with no provision for gutters or othermeans to remove the condensate at vertically spaced intervals. Thus thecondensate which collects on the upper wall portions falls to the lowerwall portions.

There is a first curve of the heat transfer resistance caused by thefree-falling film on the evaporating side of the heat transfer wall.This resistance is highest at the top of the wall and decreasesdownwardly until it becomes substantially constant over about the lowerhalf of the wall. There is a second curve of the resistance on thecondensing side of the wall; and from this curve it is apparent that ascondensate collects and flows to the lower wall portions to increase thecondensate film thickness over the lower wall portions, the resistanceon the condensing surface at the lower wall portions increases until itis quite disproportionate to that on the evaporating side of the wall.There is a third plot which is the heat transfer resistance of thealuminum wall. This resistance is relatively small an is constant overthe entire wall, and hence appears as a straight line at the lowerportion of the graph. The fourth curve (i.e., the uppermost curve)represents the total resistance.

In the present invention, by providing gutters at properly spacedintervals of l or 2 feet, the heat transfer resistance on the condensatesurface is not permitted to become disproportionate to the heatresistance on the opposite wall surface. For example, if the gutters arespaced at 2-foot intervals, the heat transfer resistance of thecondensate film that occurs within the first 2 feet of the wall would beessentially the same for each 2-foot vertical interval for the entirewall. Obviously, if the gutters were spaced at 2-foot intervals, theresistance of the condensate film would be lower yet for the entire wallsurface. Within these limits, it becomes a matter of design requirementsas to how close to make the spacing of the gutters in accordance withthe desired performance of the unit. However, when the spacing of thegutters becomes greater than about four feet, it can be seen that theheat transfer resistance of the condensate film is becoming quitedisproportionate so as to destroy the balance of heat transfercoefficients on each side of the wall and thus impair the properoperation of the unit.

It is also to be understood that numerous modifications of the processand apparatus described above may be made within the scope of theinvention. For example, the desired downward velocity of the flowingfilm on the heat transfer surface need not be achieved by an upwardextension of the heat transfer wall but can be achieved instead byforming the film from a stream which has a downward velocity imparted toit by a pump. When the film has an initial downward velocity it isespecially preferable to have a flat heat transfer wall since liquidentering a tubular channel at high velocity forms eddy currents whicherode and eventually cut through the tube.

The condensate film may be controlled in thickness by means other thanthe collection troughs described above, as for example, by weirs spacedfrom the wall by the desired distance.

it is also to be understood that the number of atmospheric evaporators,of flashing tanks and evaporative crystallizers and the dimensionsthereof may be varied within the general teachings of this invention.

lclaim:

l. A method of evaporating a liquid containing a dissolved solid whichcomprises evaporating a portion of said liquid at atmospheric pressureand at its atmospheric boiling point to precipitate a substantial amountof dissolved solid therefrom and to leave said liquid saturated withrespect to said dissolved solid, transferring said saturated liquid atits atmospheric boiling point to a reduced pressure flashing zone as thesole liquid feed thereto and evaporating additional liquid in saidflashing zone substantially adiabatically while precipitating additionaldissolved solid therefrom.

2. The method of claim 1 wherein said liquid containing a dissolvedsolid is an aqueous saline solution and wherein said evaporation atreduced pressure takes place at a temperature between about 60 C. andabout 80 C. and precipitates said dissolved solid from said solution.

3. The method of claim 1 wherein said liquid containing a dissolvedsolid is a brine containing dissolved sodium chloride and calciumsulfate, wherein said atmospheric pressure evaporation precipitates asubstantial amount of calcium sulfate therefrom and said reducedpressure evaporation precipitates additional calcium sulfate therefrom.

4. The method of claim 3 wherein said atmospheric evaporation stepevaporates at least 70 percent of the water in said brine.

5. The method of claim 3 wherein calcium sulfate is ture of about 250 F.

7. The method of claim 1 wherein said liquid containing a dissolvedsolid is a brine containing dissolved sodium chloride and calciumsulfate, wherein said atmospheric pressure evaporation is carried outwhile said brine is moved as a downflowing film on a heat transfer wallat a downward velocity of at least about 14 feet per second toprecipitate a substantial amount of calcium sulfate therefrom.

8. The method of claim 7 wherein said brine passes downwardly as aflowing film over each ofa series of at least four heat transfer walls.

9. The method of claim 7 wherein steam is removed during saidatmospheric pressure evaporation, said steam is compressed by a pressureincrement of about 16 p.s.i. to impart superheat thereto, saidcompressed steam is desuperheated while maintaining substantially all ofsaid pressure increment, said desuperheated, compressed steam is passedinto contact with the opposite side of said heat transfer wall, whereinat least a portion of said steam is condensed as a downflowing film onsaid opposite side of said heat transfer wall an wherein at least aportion of the liquid condensate in said last named film is collected atvertically spaced locations in its downward path.

10. In a method of evaporating a liquid containing a dissolved solidwherein a portion of said liquid is evaporated in at least oneevaporation zone to precipitate a substantial amount of dissolved solidtherefrom and therein said liquid, concentrated and saturated and as aslurry containing said precipitated solid, is transferred through atransfer line to a separation zone to separate precipitated solidtherefrom, the improvement which comprises limiting the amount ofprecipitation in said evaporation zone so that the viscosity of theslurry obtained is not higher than about centipoises and maintaining thevelocity of said slurry in said transfer line at at least about 8 feetper second, said liquid being evaporated in at least two successiveevaporation zones before passing to said separation zone and wherein theprecipitation in the first of said evaporation zones is limited so thatthe viscosity of the slurry obtained is not higher than about 150centipoises and the velocity in said transfer line to said secondevaporation zone is maintained at least 8 feet per second, said firstevaporation zone being maintained at atmospheric pressure and saidsecond evaporation zone being maintained at reduced pressure.

' UNITED STATES PATENT OFFICE 5 9 CERTIFICATE OF CORRECTION Patent No.3,586, 090 Dated June 22, 1971 InventorIB) George L. Henderson r- Column1, line 20, "bee' should read been It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3, line 26, insert semicolon after "invention".

Column 5, line 24, "per second" should read per second per second Column6, line 57, "filer" should read filter line 59 "lie" should read lineColumn 7, line 25, filer" should read filter Column 8, line 22, "(b)"should read (2) Column 11, line 16, "of" should read to Column 12, line30, "FIG. should read FIGS. line 69, "2-foot" should read l-foot Column1 4, line 32, "therein" should read wherein Signed and sealed this 21stday of December 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Patents

2. The method of claim 1 wherein said liquid containing a dissolvedsolid is an aqueous saline solution and wherein said evaporation atreduced pressure takes place at a temperature between about 60* C. andabout 80* C. and precipitates said dissolved solid from said solution.3. The method of claim 1 wherein said liquid containing a dissolvedsolid is a brine containing dissolved sodium chloride and calciumsulfate, wherein said atmospheric pressure evaporation precipitates asubstantial amount of calcium sulfate therefrom and said reducedpressure evaporation precipitates additional calcium sulfate therefrom.4. The method of claim 3 wherein said atmospheric evaporation stepevaporates at least 70 percent of the water in said brine.
 5. The methodof claim 3 wherein calcium sulfate is separated from residual liquidafter said reduced pressure evaporating step and wherein said residualliquid is separately processed to recover sodium chloride therefrom. 6.The method of claim 3 wherein said evaporation in said atmosphericevaporation step takes place at a final temperature of about 250* F. 7.The method of claim 1 wherein said liquid containing a dissolved solidis a brine containing dissolved sodium chloride and calcium sulfate,wherein said atmospheric pressure evaporation is carried out while saidbrine is moved as a downflowing film on a heat transfer wall at adownward velocity of at least about 14 feet per second to precipitate asubstantial amount of calcium sulfate therefrom.
 8. The method of claim7 wherein said brine passes downwardly as a flowing film over each of aseries of at least four heat transfer walls.
 9. The method of claim 7wherein steam is removed during said atmospheric pressure evaporation,said steam is compressed by a pressure increment of about 1-6 p.s.i. toimpart superheat thereto, said compressed steam is desuperheated whilemaintaining substantially all of said pressure increment, saiddesuperheated, compressed steam is passed into contact with the oppositeside of said heat transfer wall, wherein at least a portion of saidsteam is condensed as a downflowing film on said opposite side of saidheat transfer wall and wherein at least a portion of the liquidcondensate in said last named film is collected at vertically spacedlocations in its downward path.
 10. In a method of evaporating a liquidcontaining a dissolved solid wherein a portion of said liquid isevaporated in at least one evaporation zone to precipitate a substantialamount of dissolved solid therefrom and therein said liquid,concentrated and saturated and as a slurry containing said precipitatedsolid, is transferred through a transfer line to a separation zone toseparate precipitated solid therefrom, the improvement which compriseslimiting the amount of precipitation in said evaporation zone so thatthe viscosity of the slurry obtained is not higher than about 150centipoises and maintaining the velocity of said slurry in said transferline at at least about 8 feet per second, said liquid being evaporatedin at least two successive evaporation zones before passing to saidseparation zone and wherein the precipitation in the first of saidevaporation zones is limited so that the viscosity of the slurryobtained is not higher than about 150 centipoises and the velocity insaid transfer line to said second evaporation zone is maintained atleast 8 feet per second, said first evaporation zone being maintained atatmospheric pressure and said second evaporation zone being maintainedat reduced pressure.